Several procedures have been described in the art for improving the optical efficiency of pigment slurries for application to paper products or incorporation into other coating materials. Typically, these procedures involve surface treating the titanium particles present in the slurry with a secondary compound such as alumina, silica, magnesia, or the like or a combination of these. Exemplary of this art are U.S. Pat. Nos. 2,868,663, 2,671,031, 3,510,334, and 4,050,951. According to U.S. Pat. No. 2,868,663, titanium dioxide pigment can be surface treated with a metal oxide by introducing a salt of aluminum, titanium, silicon, zirconium, or a mixture thereof, precipitating the salt onto the titanium dioxide pigment and subsequently washing to remove any soluble salts. The surface treated pigment is then dried before use. U.S. Pat. No. 2,671,031 describes the use of an ionizable aluminum compound to coat pigment particles. However, again, the pigment is dried by baking at an elevated temperature prior to use. Similarly, U.S. Pat. Nos. 3,510,334 and 4,050,951 teach the production of dry coated pigments either by a dry method or a slurry method. However, in each case if a wet slurry product is to be produced, the pigment is first treated, dried and then reslurried to make the wet slurry product.
The drying step has heretofore been thought to be required when treating TiO.sub.2 particles with aluminum, titanium, silicon, zirconium, or magnesium compounds to improve optical efficiency, the drying being an essential step in surface treating the pigment's particles with the optical efficiency improving compound. Where the surface treating agent was aluminum based, the surface became treated with essentially crystalline aluminum oxide.
It has now been found that a titanium dioxide slurry with improved optical efficiency can be produced directly by blending a particulate TiO.sub.2 material with an amorphous aluminum hydroxide and water to form a wet slurry. Drying or otherwise attempting to precipitate the aluminum hydroxide as a coating on the particles is not required.
It is an object and advantage of the present invention to produce wet pigmented slurries through a simplified blending process as distinct from conventional surface treatment of TiO.sub.2 pigments.
It is a further object and advantage of the present invention to produce anatase TiO.sub.2 slurries in a more economical fashion, and to upgrade the anatase TiO.sub.2 pigment in such slurries in terms of its relative brightness.
It is a still further object and advantage of the present invention to improve the optical efficiency of wet pigment slurries. Still other objects and advantages of the present invention will become more apparent from the detailed description of the invention.